Well, one reason to look beyond local flora and fauna is because they might not do so well in the next 10 or 20 years.
So if the answer to dry spells in Sussex is switching sheep breeds from long-established local ones to others from hot and dry but otherwise completely alien Antipodean environments (rather than, say, France, just south of Sussex), I think the analysis must have been swift and shallow.
This exactly. It reads as a poorly planned decision. The move should be gradual, and there should at least be some local breed in there. Adding a bit of Pita Pinta, Fayoumi, Penedesenca (for some reason my mind always reads this breed as Pedesena or Penedescena

The wildlife is free to move wherever it likes, and I’m sure we’re all start to see more of both wanted and unwanted new species in our land, skies and water, like the lionfish here. Still the new migratory patterns and locations are not drastically different. For animals that still receive human care, like sheep and chickens, we need to take new breed introduction slow, or else it will fail. I’m sure my birds would drop dead in your winters